I’m not sure about sunlight causing these. I never used charger outdoor. I have a 13” and 15” mbp, magsafe-3 oem and 2 3rd party (lot cheaper-could be refurb? Got one from WM for $15). So far, only the OEM parts are the ones that are browning. I will go with apple explanation and I use the OEM’s as spares. One of the 3rd party seller quit working after about 8 months. It’s under warranty, so they replaced it.